1. Field of the Invention
This invention relates generally to a method for determining flow out of a closed system and, more particularly, to a method for determining flow of anode gas out of an anode sub-system in a fuel cell system, where the method determines the slope of a pressure line formed from a plurality of pressure measurements and uses the slope in a flow calculation.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electro-chemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated at the anode catalyst to generate free protons and electrons. The protons pass through the electrolyte to the cathode. The protons react with the oxygen and the electrons at the cathode catalyst to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacture and require certain conditions for effective operation.
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. For example, a typical fuel cell stack for a vehicle may have two hundred or more stacked fuel cells. The fuel cell stack receives a cathode input gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen input gas that flows into the anode side of the stack.
A fuel cell stack includes a series of bipolar plates positioned between the several MEAs in the stack, where the bipolar plates and the MEAs are positioned between two end plates. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow channels are provided on the anode side of the bipolar plates that allow the anode reactant gas to flow to the respective MEA. Cathode gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode reactant gas to flow to the respective MEA. One end plate includes anode gas flow channels, and the other end plate includes cathode gas flow channels. The bipolar plates and end plates are made of a conductive material, such as stainless steel or a conductive composite. The end plates conduct the electricity generated by the fuel cells out of the stack. The bipolar plates also include flow channels through which a cooling fluid flows.
It is necessary to accurately determine the flow rate through bleed valves, drain valves, and possibly other valves, in the anode sub-system of a fuel cell system to know when to close the particular valve, as is well understood by those skilled in the art. Traditional valve orifice models work fairly well, but are subject to part-to-part variations because the models assume an effective area of the orifice. Further, the orifice model calculation also requires a difference between an inlet and outlet pressure to determine the flow. For certain known systems, this pressure differential is on the same order of magnitude as the error of the pressure sensors, which could lead to large estimation errors.
U.S. patent application Ser. No. 12/636,276, titled Injector Flow Measurement for Fuel Cell Applications, filed Dec. 11, 2009, assigned to the assignee of this application and herein incorporated by reference, discloses a method for determining flow through a valve in a fuel cell system. An anode sub-system pressure is measured just before an injector pulse and just after the injector pulse, and a difference between the pressures is determined. This pressure difference, the volume of the anode sub-system, the ideal gas constant, the anode sub-system temperature, the fuel consumed from the reaction of the fuel cell stack during the injection event and the fuel cross-over through membranes in the fuel cells of the fuel cell stack are used to determine flow through a valve. The determination of the flow in the '276 application uses two pressure measurements that could be affected by noise in the pressure readings and provides no feedback as to the validity of the pressure measurements.